[Spring] Merging StiffSpringForceField and its parent

[bakpaul]

StiffSpringForceField is basically an extension of SpringForcefield adding implicit solving. The only added value comes from the ability to define the springs using added data indices1, indices2 and lengths. This mechanism need to also be merged with the current one available in SpringForceField.

Related to https://github.com/sofa-framework/sofa/issues/248

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